1. Field of the Invention
The present invention generally relates to the field of transconductors. More specifically, the present invention relates to transconductors likely to be used in multimode devices, that is, capable of processing signals having different frequency characteristics.
So-called class A non-linear transconductors and so-called class AB transconductors are more specifically considered in the present description.
2. Discussion of the Related Art
Class A or class AB transconductors are used to perform many functions such as, in particular, amplifications or signal mixings. For example, transconductors are used in chains of transmission or reception of signals to implement, in particular, so-called low-noise receive amplifiers (LNA), so-called power transmission amplifiers (PA) or mixers.
The type—class A or class AB—of the transconductor used to implement such function depends on the application. Consider for example the case of transmission chains in the field of radiotelephony or mobile telephony. In this field, there exist different standards especially characterized by different frequency ranges, among which standards UMTS or WCDMA, of a frequency on the order of 2.16 Hz, standard GSM of a frequency of 900 MHz, or standard DCS of a frequency of 1.8 GHz.
FIG. 1A schematically illustrates the structure of a transconductor 1. Such a transconductor 1 includes an NPN-type bipolar transistor 2. Base 3 of transistor 2 forms a first input/output terminal of the transconductor. Base 3 receives an input signal IN, for example, in mobile telephony, a radiofrequency signal. Emitter 4 of transistor 2 is connected to a low-voltage reference line or ground GND, via a series connection of an impedance 5 and of a constant D.C. current source 6. Collector 7 of transistor 2 forms a second input/output terminal of the transconductor and provides output signal OUT of transconductor 1. Collector 7 is connected to any device, illustrated by an input/output terminal 8, providing a signal to be mixed with input signal IN. For example, device 8 provides a signal enabling switching input signal IN, or a carrier signal to be modulated by input signal IN. Device 8 is not necessarily unique. It may be an intermediary stage of a mixer of several signals, for acquiring one of the signals to be mixed.
FIGS. 1B and 1C respectively illustrate characteristics of transconductance gain G and of current I as a function of the level of voltage input V in the transconductor of FIG. 1A. Transconductance gain G, which is proportional to the value of the bias current of transistor 2, is a constant g0 for low levels of base-emitter voltage V across transistor 2. Beyond a given threshold V0, gain G decreases. Short of threshold V0, the dynamic output current IOUT then is, as illustrated in FIG. 1C, proportional to base-emitter voltage V and limited by value IDC of D.C. current source 6. Beyond threshold V0, the behavior of the class A transconductor is poorly defined. The input signals are thus limited to those for which the input voltage is smaller than V0.
FIG. 2A schematically illustrates the structure of a class AB transconductor 20. FIGS. 2B and 2C, which are homologous to FIGS. 1B and 1C, illustrate the gain and current characteristics according to the input voltage of transconductor 20.
Transconductor 20 includes an NPN-type bipolar transistor 21, base 22 of which forms a first input/output terminal, receiving a signal IN, for example, a radiofrequency signal, collector 23 of which forms an output terminal of a current OUT and emitter 24 of which is degenerated, that is, connected to a reference supply GND by an impedance 25. Further, base 22 is connected to a current bias source 26 by a resistor 27. As previously for class A transconductor 1 of FIG. 1A, collector 23 may be connected to an input device 28 of a signal to be mixed or of an amplification order or of a carrier signal or other.
Transconductor 20 exhibits an exponential characteristic of gain G according to input base-emitter voltage V, illustrated in FIG. 2B. This enables, as illustrated in FIG. 2C, obtaining a static component (or mean current) IOUT  of the output signal current which varies as a function of the input signal. The dynamic component of output signal current Iout is then no longer limited by the bias signal, but follows, or even exceeds, the mean current.
In the example of application to telephony, to enable a user to keep a given device when a standard changes, one mixer per frequency range must be provided for each function. Such a solution goes against the miniaturization of portable devices.
It could then be devised to use class A mixers formed of transconductors similar to transconductor 1 of FIG. 1A, which would be forced to have a linear component by imposing a constant current (source 6) sufficiently high for transistor 2 to operate in linear state. Such a solution would have many disadvantages. In particular, such mixers would have a relatively poor linearity as compared to class AB mixers formed of class AB transconductors. Further, this would be obtained at the expense of high power consumption. Such a power consumption would then also exist for small signals although it is not necessary. This would be particularly disadvantageous due to the large number of transconductors used in a multimode transceiver device. The required power supplies would then become bulky and impose at best frequent recharges, which is incompatible with the mobile character of the device. Further, the high power consumption, useless in the case of small signals, would impose additional dissipation constraints in the form of thermal power.